A variety of touch probes are used with coordinate positioning machines, such as coordinate measuring machines or machine tools to measure a position on or along a surface, e.g., a workpiece surface. There are many coordinate positioning machine designs, but such machines typically include a moveable arm to which the probe is attached. The arm is supported for movement relative to a datum, such as a platform or table on which a workpiece is supported. This allows an operator to use the coordinate positioning machine in combination with the probe to determine whether certain positions on the workpiece are at their proper location relative to the structure on which the workpiece is supported.
Some probes, such as touch probes, are designed to produce a signal when the stylus carried by the probe contacts a surface. This type of probe includes a fixed structure that is mounted to the moveable arm of the coordinate positioning machine. A stylus structure is supported on the fixed structure at several locations. For example, the stylus structure may include three balls biased against a contact surface of the fixed structure. In some designs, the balls and contact surfaces are part of a circuit which is broken when the stylus contacts an object and one of the balls is forced away from the contact surface. When the circuit is broken, a signal is provided to indicate the contact between the stylus and an object. In other probes, the stylus support structure is connected to a strain sensor that provides a signal when strain is induced via contact of the stylus with an object.
With these types of probes, it is critical that the stylus be moved back to a precise and repeatable rest position after contact with an object. Sometimes grooves may be formed in the contact surface to assist in precisely reseating the balls after deflection of the stylus. Without this precise reseating of the stylus support structure, the position of the stylus would be different for each subsequent measurement of position, and errors would be introduced into the measurement.
The accuracy of these probes depends on maintaining a mechanically repeatable rest position of the stylus support structure This is often difficult, because wear can result due to the repeated contact between the balls and the contact surface and due to the electrical current that can degrade electrical contacts. Also, slow, careful contact is necessary to obtain an accurate measurement of the point at which the stylus contacted the object. In fact, the required sensitivity may be so great it is sometimes necessary to adjust the amount of force biasing the stylus support structure back to its rest position when different styli are interchanged.
Attempts have been made to design probes able to measure movement of the stylus after contact with an object. If this movement can be measured, then it is a straightforward mathematical calculation to determine the location of the stylus prior to movement and thus the precise point of contact with the object being measured, obviating the need for a precise mechanically repeatable rest position. For example, in U.S. Pat. No. 5,390,424, an analog probe is disclosed that measures the movement of a stylus along the three linear axes commonly known as the x-axis, y-axis and z-axis. This probe includes a stylus-supporting assembly that has three slideable members supported on air bearings for movement along the x, y and z axes. The movement in the x, y or z directions is measured by opto-electronic transducers that each comprise a scale and a readhead. Thus, movement of the stylus after contacting the object being measured can be sensed in three linear directions.
The probe described above works well in many applications, but movement of the stylus supporting member must be accurately constrained to movement along the three linear axes. Any other movement of the stylus would be detrimental to the accuracy of the probe. However, a stylus, like any object, potentially can be moved with six degrees of freedom. The stylus can be moved along the linear axes x, y and z, but it can also be moved along the rotational axes, commonly known as the a-axis, b-axis, and c-axis. The a, b and c axes represent rotational movement of an object about the x, y and z axes respectively.
Therefore, it would be advantageous to avoid these limitations and to provide a multi-axis continuous probe potentially able to track the movement of its stylus along all six axes.